The present invention relates to a novel method and apparatus for monitoring the level of oxygen in a feed stream or within the atmosphere of a defined space such as an oxygen chamber, oxygen tent, a room, or the like.
In the past there have been a number of procedures and apparatus proposed for the monitoring or analysis or sampling of gases. These include electrochemical methods, mass spectrometry methods, CHEMFET devices, charger flow transistors, gas chromatographic and other colorimetric procedures. In general, these involved extremely expensive and sophisticated equipment and techniques or mechanisms which were not truly reversible and/or indefinitely continuous. In most cases, they relied on a chemical reaction by the gas which would provide a corresponding change in pH thereby triggering a color change in an indicator as a colorimetric chemical reaction which was not quickly and fully reversible. Also, such systems were obviously subject to the vagaries of other gases which might be present particularly the relative amount of humidity present.
The earliest systems for monitoring of gases generally related to gases such as carbon dioxide, hydrogen sulfide, halogens and the like. These systems were colorimetric in nature, but the colorimetric reaction was not immediately and completely self reversing in response to a reversal of the change in concentration of the gas being monitored.
As examples of one type of system taught by the prior art, mention might be made of U.S. Pat. No. 3,754,867 Karl R. Guenther, in which the carbon dioxide content of ambient air is monitored using a thin layer of chemical which will absorb carbon dioxide forming an acid which will provide a change in pH. An indicator present in the film changes color. The system circumvents problems of humidity by using an ionizing solvent having a vapor pressure in the range of 0-10 mm at temperatures up to 150.degree. F., and compatible with the other components of the system.
Another method is proposed by U.S. Pat. No. 3,114,610 to Gafford et al., using very sophisicated analyzing equipment to measure the particular presence of a constituent of a gas, which constituent produces acidic or basic solutions. Again, basically a one-way system in which the sample must be either neutralized, or the indicator replaced, or the instrument recalibrated before further sampling can continue.
U.S. Pat. No. 2,232,622 to Moses et al., and U.S. Pat. No. 2,741,544 to Chaikin et al., provide an alternate method in which continuous sampling is possible over a finite period of time. Moses et al., relates to the monitoring of hydrogen sulfide, and Chaikin et al., relates to the apparatus of fluoride analysis. They are, however, very similar methods, in that continuous analysis over a finite period of time is achieved by winding forward a continuous strip of tape impregnated with the indicator. As with Guenther and Gafford, however, the system relies on a change in pH to trigger a color change in an indicator.
All of the foregoing systems have certain basic limitations. They can only measure gases which provide an acidic or basic solution such as carbon dioxide, hydrogen sulfide, halogen, or the like; and they are operable, at best, intermittently or over a relatively finite period of time. In addition, those which do provide for some measure of continuous monitoring, such as Moses et al., involve very cumbersome and relatively expensive apparatus, such as a drive motor and the like.
The range of oxygen detection methods is large but generally very sophisticated and more expensive than those described above and includes such diverse means as electrochemical reactions and cells, optical fiber monitors based on fluorescence quenching of dyes or colorimetric oxygen reactions, CHEMFETS and charge-flow transistor devices, anaerobic bacterial activity, mass spectrometry, gas chromatography and the addition of odorants of other detectable trace gas additives to the oxygen supply. However, use of most such techniques is far from commercialization, while others are suitable only for certain limited applications. None of these techniques provide an inexpensive continuous simple procedure for in-line monitoring of oxygen level in a fuel stream or enclosed area.
The Clark cell [L. C. Clark, Jr., Trans. Amer. Soc. Artif. Intern. Organs, 2, 41-48 (1965)] is the most commonly used electrometric oxygen sensor available today. It is based on polarographic principles by which, for a given applied voltage, the current between two electrodes is directly proportional to the oxygen partial pressure in the environment.
A very similar polarographic monitor has also been developed by Hersch [W. Bahmet and P. A. Hersch, Anal. Chem., 43, 803 (1971) and P. A. Hersch, Amer. Lab, Aug. 1973, p. 29] and is based on the linear variation of the limiting current attainable from a cadmium-air cell when the partial pressure of oxygen is varied. There are two very major problems with such electrochemical methods, they depend on the precise maintenance of solution concentration, and they depend upon a kinetically limited gas liquid equilibrium system. One can speculate optical methods since these methods could theoretically be based on any colorimetric oxygen reaction.
Mass spectrometry and gas chromatography, however, are the methods conventionally used for the quantitative and qualitative analysis of gases, and could easily be adapted to oxygen monitoring. A major consideration in their use, however, would be their relative cost and size. A detector based specifically on the paramagnetic properties of oxygen is also conceivable, but seems even less promising than mass spectrometry or gas chromatography on the basis of cost, size and versatility. Thus, simple optical systems are purely speculative, while instrumental procedures are too complex and too expensive.
Transistor devices have also been suggested. CHEMFET devices have been proposed for monitoring systems. Use of these chemically sensitive field effect transistor devices [J. Janata and R. H. Huber, in "Ion-Selective Electrodes", Analytical Chemistry, Vol. 2, H. Freiser, ed., Plenum Press, New York, 1980, pp. 124-6] is predicated on the measurement of changes in the source/drain current passing through a transistor due to variations in the electric field in the gate region of the device. The observed changes in current could, for example, result from the absorption of oxygen on, or its reaction with, material, in the gate region of the device.
Charge-flow transistors have also been suggested. Application of these devices [S. L. Garverick and S. D. Senturia, IEEE Trans. Electron Dev., 29, 90 (1982)] involves the measurement of the change in admittance (AC conductance) of a transistor resulting from the adsorption of a given species (e.g. O.sub.2) on, or its reaction with, a resistive material placed in the gate region of the device. The admittance of the device is directly related to the time delay observed between the application of a gate-to-source voltage and the initiation of the source-to-drain current. Both CHEMFET devices and charge-flow transistors tend to be very complex systems overall, and yet are very unreliable.
None of the teachings heretofore available provide a truly inexpensive and completely reliable apparatus andor method by which the oxygen content of a gas or atmosphere can be continuously and reversibly monitored over an indefinite period of time using nondepletable materials and, insofar as the monitoring element, no moving parts. It will be appreciated that a serious need exists to monitor the oxygen content of a gas feed stream or the atmosphere within a container, chamber, room, or the like, to maintain continuous monitoring with instantaneous warning in the event of an undue pressure drop. Such systems and apparatus would have particular utility and applicability in medical applications, such as monitoring the oxygen feed to a patient and/or the oxygen content of the atmosphere within an oxygen tent or room. Such monitoring is now possible, if at all, only using extremely cumbersome and expensive equipment.